The present invention relates generally to control circuits for induction motors, and more particularly, to a power converter for use with induction motors that provides for emergency operation and fail soft operating characteristics.
A block diagram of a prior art power converter structure is shown in FIG. 1, and is illustrated for comparative purposes. In normal operating mode, this circuit converts DC input power to three-phase AC power with regulated output voltage or currents, to control and drive a three-phase motor, such as an induction motor or brushless DC motor. With reference to FIG. 1, a fail soft mode of operation of this converter is achieved by connecting the neutral point of a three-phase Y-connected motor by way of a triac to a voltage that is midway between +DC and -DC input buses. However, the disadvantages of the circuit include the following: capacitors C2 and C3 carry very large currents when operating and must be physically large; resistors R1 and R2 cause additional power losses; emergency-mode operation is only possible if a power switch fails in an open circuit condition, not in a short-circuit condition.
Consequently, it is an objective of the present invention to provide for a power converter for use with a three phase induction motor that overcomes the above disadvantages, and provides for fail soft operation in the event of any type of failure of the power switches.